Sir Ian Blair defied a barrage of calls to resign last night after Scotland Yard was found guilty over the shooting of Jean Charles de Menezes.

The controversial Metropolitan Police Commissioner was under mounting pressure to take responsibility for the "catastrophic" series of errors which resulted in his officers killing the innocent Brazilian.

Yesterday, following a fiveweek trial, an Old Bailey jury took just four hours unanimously to convict the Met of breaching health and safety laws and putting the public at risk. The force was fined £175,000 and ordered to pay £385,000 costs by an Old Bailey judge.

Despite this, no individual officers have been charged with criminal offences over the case.

A number have already been told they will not even face any disciplinary charges, meaning that nobody might ever be held personally accountable for the shambolic operation which resulted in Mr de Menezes being shot seven times after being mistaken for a terrorist.

Relatives of Mr de Menezes hailed the verdict but accused Met lawyers of trying to "blacken" his name in a desperate attempt to win the case.

Victim: Jean Charles de Menezes was shot dead at Stockwell Tube station in July 2005

Defence barrister Ronald Thwaites QC claimed during the trial that he behaved suspiciously because he was an illegal immigrant with cocaine in his system.

Even after the guilty verdict, Sir Ian defended the alleged slur yesterday, saying the Met's legal team had pointed out these factors to "explain to the jury some of the reasons why Mr de Menezes acted as he did when confronted by armed police".

At her home in Brazil last night, his mother Maria said: "Sir Ian should now think about what his men have done to my son and consider resigning."

Her call was echoed by senior politicians who demanded that Sir Ian, whose two-and-a -half years as head of the Met have been marred by a series of gaffes and errors of judgment, should quit following the £3.5million trial.

Shadow Home Secretary David Davis said: "The failures were systemic, falling within the clear responsibility of the Commissioner of the Met Police. His position is now untenable, in light of these findings and the overriding need to restore public confidence."

Liberal Democrat home affairs spokesman Nick Clegg said: "This guilty verdict makes it unavoidable that Ian Blair should take responsibility on behalf of his whole organisation and resign."

Although the Commissioner insisted he would not stand down, his survival prospects looked bleak last night after an emergency meeting of the Metropolitan Police Authority was called by Tory member Richard Barnes to discuss the crisis.

Sources said Sir Ian would face a no confidence motion at the meeting, which is due in the next seven days. The Daily Mail understands that at least ten of the MPA's 23 members have already pledged to vote against Sir Ian while several others plan to abstain.

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Cressida Dick: The jury cleared her of any personal responsibility

MPA member Damian Hockney said: "Today the Metropolitan Police have been found guilty, but nobody is to be held accountable. This is a ludicrous state of affairs."

Sir Ian, bolstered by statements of support from Prime Minister Gordon Brown and Home Secretary Jacqui Smith who said they had full confidence in him, expressed his "deep regret" for the "tragedy" of the 27-year-old electrician.

But he insisted it was an "isolated breach" under "extraordinary circumstances" and gave his backing for all officers involved in counter-terrorism - in particular Deputy Assistant Commissioner Cressida Dick, who was in overall charge of the botched operation.

Outside the Old Bailey, Sir Ian indicated he would have "considered his position" if the court case had revealed "systemic failings" in the Met.

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Trial: Prosecutors set out 19 alleged failings in the police operation leading up to the shooting of De Menezes at Stockwell tube station on July 22, 2005

But he said: "This case thus provides no evidence at all of systematic failure by the Metropolitan Police."

He added: "I am not going to consider my position about the events, as the judge described, of a single day in extraordinary circumstances."

Apologising to the innocent Brazilian's family, Sir Ian said: "The death of Jean Charles de Menezes was a tragedy. Once more I express my deep regrets for his death."

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Shooting: Mr de Menezes was shot seven times by specialist firearms officers

An inquest into his death is due to take place next year and the Met is eventually expected to pay up to £ 1million in compensation to the family of Mr de Menezes.

Further damaging details about the shooting are due to be revealed in the next few days in a police watchdog report on the shooting.

Last night Ken Jones, president of the Association of Chief Police Officers, warned the conviction could have major consequences. He called for a debate on whether the Health and Safety at Work Act should be changed to take into account the special nature of police operations.

The jury heard that Mr de Menezes was shot seven times by specialist firearms officers at Stockwell Tube station in South London after being mistaken for failed suicide bomber Hussain Osman.

Memorial: Flowers and tributes poured in for de Menezes after his shooting

Prosecutors set out 19 alleged failings in the police operation in the hours leading up to the shooting on July 22, 2005. During the trial, they claimed that "fundamental failures" at all levels led to the death of Mr de Menezes.

But in a rider to the verdict, the jury cleared Cressida Dick of personal responsibility. Trial judge Mr Justice Henriques said: "I am deliberately not going to name any individual as having failed.

"This was a corporate failing with a number of failures contributing to the ultimate tragedy."